examples/open_world_city/Assets/Scripts/VehicleController.cs

using UnityEngine;

namespace UnityAgent
{
    /// <summary>
    /// Arcade car physics: motor torque, steering, braking and a simple
    /// four-wheel raycast suspension. Place this on a root GameObject with
    /// a Rigidbody and four wheel transforms.
    /// </summary>
    [RequireComponent(typeof(Rigidbody))]
    public class VehicleController : MonoBehaviour
    {
        [System.Serializable]
        public class Wheel
        {
            public Transform transform;
            public bool isSteering;
            public bool isPowered;
            public bool isBraking;
            [HideInInspector] public bool isGrounded;
            [HideInInspector] public float suspensionOffset;
        }

        [Header("Wheels")]
        public Wheel[] wheels = new Wheel[4];

        [Header("Engine")]
        public float motorTorque = 1500f;
        public float brakeTorque = 3000f;
        public float maxSteerAngle = 30f;
        public float maxSpeedKph = 180f;

        [Header("Suspension")]
        public float suspensionRestLength = 0.5f;
        public float suspensionSpring = 35000f;
        public float suspensionDamper = 4500f;
        public float wheelRadius = 0.35f;
        public LayerMask wheelMask = ~0;

        [Header("Aerodynamics")]
        public float downforce = 100f;
        public float centerOfMassOffset = -0.5f;

        private Rigidbody _rb;
        private float _throttle;
        private float _steer;
        private float _brake;

        private void Awake()
        {
            _rb = GetComponent<Rigidbody>();
            _rb.centerOfMass += Vector3.up * centerOfMassOffset;
            _rb.interpolation = RigidbodyInterpolation.Interpolate;
        }

        private void Update()
        {
            _throttle = Input.GetAxis("Vertical");
            _steer = Input.GetAxis("Horizontal");
            _brake = Input.GetKey(KeyCode.Space) ? 1f : 0f;
        }

        private void FixedUpdate()
        {
            UpdateWheels();
            ApplySuspension();
            ApplyDrive();
            ApplyDownforce();
        }

        private void UpdateWheels()
        {
            foreach (var w in wheels)
            {
                if (w.transform == null) continue;
                Vector3 origin = w.transform.position;
                bool hit = Physics.Raycast(
                    origin, -transform.up, out RaycastHit r,
                    suspensionRestLength + wheelRadius, wheelMask,
                    QueryTriggerInteraction.Ignore);
                w.isGrounded = hit;
                w.suspensionOffset = hit ? r.distance - wheelRadius : suspensionRestLength;
            }
        }

        private void ApplySuspension()
        {
            foreach (var w in wheels)
            {
                if (!w.isGrounded) continue;
                float force = (suspensionRestLength - w.suspensionOffset) * suspensionSpring;
                Vector3 velocity = _rb.GetPointVelocity(w.transform.position);
                float upwardVel = Vector3.Dot(transform.up, velocity);
                force -= upwardVel * suspensionDamper;
                _rb.AddForceAtPosition(transform.up * Mathf.Max(0f, force), w.transform.position);
            }
        }

        private void ApplyDrive()
        {
            float kph = _rb.velocity.magnitude * 3.6f;
            float speedFactor = Mathf.Clamp01(1f - kph / maxSpeedKph);

            foreach (var w in wheels)
            {
                if (!w.isGrounded) continue;
                if (w.isPowered)
                {
                    Vector3 force = transform.forward * (_throttle * motorTorque * speedFactor);
                    _rb.AddForceAtPosition(force, w.transform.position, ForceMode.Force);
                }
                if (w.isBraking && _brake > 0.01f)
                {
                    Vector3 brakeForce = -transform.forward * (_brake * brakeTorque);
                    _rb.AddForceAtPosition(brakeForce, w.transform.position, ForceMode.Force);
                }
                if (w.isSteering)
                {
                    // Rotate the visual wheel and apply a small yaw torque.
                    w.transform.localRotation = Quaternion.Euler(0f, _steer * maxSteerAngle, 0f);
                }
            }
            // Steering yaw torque.
            if (Mathf.Abs(_steer) > 0.01f)
            {
                float turn = _steer * maxSteerAngle * 0.02f;
                _rb.AddTorque(transform.up * turn * (_rb.velocity.magnitude + 1f), ForceMode.Force);
            }
        }

        private void ApplyDownforce()
        {
            _rb.AddForce(-transform.up * downforce * _rb.velocity.magnitude);
        }
    }
}